Method for forming quantum dot, and quantum semiconductor device and method for fabricating the same

ABSTRACT

The method for forming a quantum dot according to the present invention comprises the step of forming an oxide in a dot-shape on the surface of a semiconductor substrate  10 , the step of removing the oxide to form a concavity  16  in the position from which the oxide has been removed, and the step of growing a semiconductor layer  18  on the semiconductor substrate with the concavity formed in to form a quantum dot  20  of the semiconductor layer in the concavity. The concavity is formed in the semiconductor substrate by forming the oxide dot in the surface of the semiconductor substrate and removing the oxide, whereby the concavity can be formed precisely in a prescribed position and in a prescribed size. The quantum dot is grown in such a concavity, whereby the quantum dot can have good quality and can be formed in a prescribed position and in a prescribed size.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a division of U.S. application Ser. No. 10/991,971,filed Nov. 19, 2004, which is a Continuation of InternationalApplication No. PCT/JP03/06250, with an international filing date of May20, 2003, which designated the United States.

TECHNICAL FIELD

The present invention relates to a method for forming a quantum dotwhich can form a quantum dot in a prescribed position and in aprescribed size, and a quantum semiconductor device comprising a quantumdot and a method for fabricating the quantum semiconductor device.

BACKGROUND ART

Conventionally, as a method for forming quantum dots, the forming methodbased on Stranski-Krastanow mode (S-K mode) is known.

S-K mode is a mode in which a semiconductor crystal to be epitaxiallygrown grows two-dimensionally (film growth) at the start of the growthand grows three-dimensionally on the stage where the semiconductorcrystal has exceeded the elastic limit of the film. A film having alarger lattice constant than the base material is epitaxially grown,whereby quantum dots of three-dimensionally grown islands areself-assembled.

S-K mode can easily make quantum dots self-assembled, and is widely usedin the field of photosemiconductor devices, etc.

Recently, techniques in the fields of quantum information and quantumcomputation attract a great deal of attention. In these fields it isvery important to form a quantum dot in a prescribed position in aprescribed size.

However, the conventional method for forming quantum dots describedabove makes positions and sizes of the formed quantum dots random.

As techniques of controlling positions for quantum dots to be formed in,the following techniques have been proposed.

For example, Japanese published unexamined patent application No.2000-315654, the specification of U.S. Pat. No. 5,229,320 and Appl.Phys. Lett., Vol. 76, No. 2, p. 167-169, (2000) propose techniques ofcontrolling positions for quantum dots to be formed in by forming inadvance by means of electron beams concavities in the surface of asemiconductor substrate in positions for quantum dots to be formed in.When a semiconductor layer is grown on a semiconductor substrate withconcavities formed in, the semiconductor layer tends to grow at a higherrate in the concavities, whereby quantum dots can be formed in theconcavities.

Appl. Phys. Lett., Vol. 75, No. 22, p. 3488-3490, (1999) and Phys. Stat.Sol. (b) 224, No. 2, p. 521-525, (2001) propose a technique of formingin advance by means of an STM (Scanning Tunneling Microscope) depositsin positions for quantum dots to be formed in to thereby control thepositions for the quantum dots to be formed in. When a semiconductorlayer is grown on a substrate with deposits formed on, concavities areformed in the semiconductor layer surface above the deposits. Whenanother semiconductor layer is grown on the semiconductor layer with theconcavities formed in, quantum dots are formed in the concavitiesbecause said another semiconductor layer grows in the concavities at afaster growth rate.

Appl. Phys. Lett., Vol. 77, No. 16, p. 2607-2609, (2000) proposes atechnique of forming trenches in the surface of a semiconductorsubstrate by means of an AFM (Atomic Force Microscope) to form quantumdots in the trenches.

However, in the above-described technique of forming the concavities bymeans of electron beams, the surface of the semiconductor substrate iscontaminated with carbon when the concavities are formed by means ofelectron beams, which makes it difficult to form quantum dots of goodquality.

In the above-described technique of controlling positions of quantumdots by using an STM, crystal defects take place in the semiconductorlayer with the concavities formed in. It is difficult to form quantumdots of good quality on the semiconductor layer having the crystaldefects. Furthermore, the diameter of the concavities are somewhatenlarged, and it is difficult to form fine quantum dots of a below 40 nm(including 40 nm)-diameter.

In the technique of controlling positions of quantum dots by using anAFM, the trenches are formed in the substrate surface by mechanicallyscraping the substrate surface with the probe of the AFM. Therefore, theconfigurations of the trenches are non-uniform. It is difficult to formquantum dots in a uniform size. The trenches are formed by mechanicallyscraping the substrate surface, which causes crystal defects in thetrenches. It is difficult to form quantum dots of good quality withoutcrystal defects in the trenches with such crystal defects.

Recently techniques in the fields of quantum information and quantumcomputation attract a great deal of attention as described above, andpossibilities of applications of quantum dots attract attention.However, for further fundamental studies and application development ofquantum dots, various technical barriers which have to be overcome arepresent.

For example, quantum dots have small sizes, and the quantum dotsself-assembled by using S-K mode are distributed at random. No methodthat makes the self-assembled quantum dots distributed at randomelectrically accessible has been so far proposed.

On the other hand, conductor pads of an about 100 nm-size can be formedby using processes, such as electron beam lithography, reactive ionetching, etc., which can make submicron processing. Accordingly, in acase that the density of quantum dots is very low, it will be possibleto form an electrode above a single quantum dot by using such process.That is, electrodes are formed suitably above the regions where quantumdots are expected to have been formed, by using a process which can makesubmicron processing. Then, whether or not the quantum dots are presentbelow the electrodes is checked. The electrode formation and thefollowing check are repeated many times, whereby the presence of asingle quantum dot below the electrode could be often found. However, itis almost impossible to capture a single quantum dot in a case that thedensity of the quantum dots is high. Such method depending on theprobability is inefficient to fabricate devices.

Japanese published unexamined patent application No. Hei 07-297381(1995) proposes a method of electrically accessing quantum dots by meansof electrodes in the form of a fine probe shape. However, in this case,in order to arrange electrodes above quantum dots, a special process forforming the quantum dots in an array or others is necessary. In PatentReference 1, electric fields of the probe-shaped electrodes will bedistributed far more widely than a size of the quantum dots.Accordingly, the electric fields of the electrode arranged above acertain quantum dot will influence quantum dots adjacent to said certainquantum dot.

As described above, the conventional techniques have established nomethod of making accurate electric access to each of quantum dots whichhave self-assembled at random.

It is very significant in various aspects of the fundamental studies,application developments, etc. of quantum dots to realize the accurateelectric access to each of self-assembled quantum dots.

An object of the present invention is to provide a method for forming aquantum dot which can form a quantum dot of good-quality in a prescribedposition and in a prescribed size.

Another object of the present invention is to provide a quantumsemiconductor device which can make accurate electric access to aquantum dot and a method for fabricating the quantum semiconductordevice.

DISCLOSURE OF INVENTION

The above-described object is achieved by a method for forming a quantumdot comprising the steps of: forming an oxide in a dot-shape on asurface of a semiconductor substrate; removing the oxide to form aconcavity in a position where the oxide has been removed; and growing asemiconductor layer on the semiconductor substrate with the concavityformed in to form a quantum dot of the semiconductor layer in theconcavity.

The above-described object is also achieved by a quantum semiconductordevice comprising: a quantum dot formed on a semiconductor substrate; asemiconductor layer formed, burying the quantum dot; and an electrodeformed by self-alignment above a position where strains are generated inthe semiconductor layer because of the quantum dot. Accordingly, thequantum dots can be accurately electrically accessible via suchelectrodes, and the quantum dots can be electrically accessibleindependently of each other.

The above-described object is also achieved by a quantum semiconductordevice comprising: a quantum dot formed on a semiconductor substrate; asemiconductor layer formed, burying the quantum dot; and an electrodeformed in a concavity formed in a surface of the semiconductor layer ina position above the quantum dot. Accordingly, the quantum dots can beaccurately electrically accessible via such electrodes, and the quantumdots can be electrically accessible independently of each other.

The above-described object is also achieved by a quantum semiconductordevice comprising: a quantum dot formed on a semiconductor substrate; afirst semiconductor layer formed, burying the quantum dot; asemiconductor dot formed on the first semiconductor layer in a positionabove the quantum dot; a dot-shaped oxide formed of the partiallyoxidized semiconductor dot; a second semiconductor layer formed, buryingthe semiconductor dot; and an electrode formed in a concavity formed ina surface of the second semiconductor layer in a position above thedot-shaped oxide.

The above-described object is also achieved by a method for fabricatinga quantum semiconductor device comprising the steps of: forming aquantum dot on a semiconductor substrate; forming a semiconductor layer,burying the quantum dots; and forming an electrode by self-alignmentabove a position where strains are generated in the semiconductor layerbecause of the quantum dot.

The above-described object is also achieved by a method for fabricatinga quantum semiconductor device comprising the steps of: forming aquantum dot on a semiconductor substrate; forming a semiconductor layer,burying the quantum dot; forming a semiconductor dot on thesemiconductor layer in a position above the quantum dot; oxidizing thesemiconductor dot and the semiconductor layer immediately below thesemiconductor dot to form a dot-shaped oxide partially buried in thesemiconductor layer; removing the dot-shaped oxide to form a concavityin a surface of the semiconductor layer; and forming an electrode in theconcavity formed in the surface of the semiconductor layer.

The above-described object is also achieved by a method for fabricatinga quantum semiconductor device comprising the steps of: forming aquantum dot on a semiconductor substrate; forming a first semiconductorlayer, burying the quantum dot; forming a semiconductor dot on the firstsemiconductor layer in a position above the quantum dot; oxidizing apart of the semiconductor dot to form a dot-shaped oxide of thepartially oxidized semiconductor dot; forming a second semiconductorlayer, burying the dot-shaped oxide and the semiconductor dot with aconcavity formed in a surface in a position above the dot-shaped oxide,and forming an electrode in the concavity formed in the surface of thesecond semiconductor layer.

According to the present invention, the concavity is formed in thesemiconductor substrate by forming the dot-shaped on the surface of thesemiconductor substrate by AFM oxidation and removing the oxide, wherebythe concavity can be formed precisely in a prescribed position and in aprescribed size. In such a concavity, the quantum dot is grown, wherebythe quantum dot can have good quality and can be formed in a prescribedposition and in a prescribed size. According to the present invention,the quantum dot of good quality can be formed in a prescribed positionand in a prescribed size, whereby quantum dots which are usable in thefields of quantum information and quantum computation, etc. can beformed.

According to the present invention, on the semiconductor substrate, thequantum dot is formed, the semiconductor layer is formed, burying thequantum dot, the electrode is formed by self-alignment above theposition where strains are generated in the semiconductor layer becauseof the quantum dot, whereby the electrodes can be formed accuratelyabove the respective quantum dots. Accordingly, the quantum dots can beaccurately electrically accessible via such electrodes, and the quantumdots can be electrically accessible independently of each other.

According to the present invention, on the semiconductor substrate, thequantum dot is formed, the semiconductor layer is formed, burying thequantum dot, the semiconductor dot is formed on the semiconductor layerin the position above the quantum dot, and the semiconductor dot and thesemiconductor layer immediately below the semiconductor dot areoxidized, whereby the dot-shaped oxide partially buried in thesemiconductor layer is formed, and the dot-shaped oxide is removed tothereby form the concavity in the surface of the semiconductor layer,and the electrode is formed in the concavity formed in the surface ofthe semiconductor layer, whereby the electrodes can be formed accuratelyabove the respective quantum dots. Accordingly, the quantum dots can beaccurately electrically accessible via such electrodes, and the quantumdots can be electrically accessible independently of each other.

Furthermore, according to the present invention, on the semiconductorsubstrate, the quantum dot is formed, a first semiconductor layer isformed, burying the quantum dot, the semiconductor dot is formed on thefirst semiconductor layer in the position above the quantum dot, a partof the semiconductor dot is oxidized to form a dot-shaped oxide of thepartially oxidized semiconductor dot, a second semiconductor layer isformed, burying the dot-shaped oxide and the semiconductor dot so thatthe concavity is formed in the surface in the position above thedot-shaped oxide, and the electrode is formed in the concavity formed inthe surface of the second semiconductor layer, whereby the electrodescan be formed accurately above the respective quantum dots. Accordingly,the quantum dots can be made accurately electrically accessible via suchelectrodes, and the quantum dots can be made electrically accessibleindependently of each other.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a view illustrating a step of a method for forming a quantumdot according to a first embodiment of the present invention (Part 1).

FIG. 2 is an AFM image of a state that the dot-shaped oxides are formedon the surface of the semiconductor substrate.

FIG. 3 is views illustrating steps of the method for forming the quantumdot according to the first embodiment of the present invention (Part 2).

FIG. 4 is an AFM image of a state that the oxides have been removed.

FIG. 5 is an AFM image of a state that the quantum dots are formed.

FIG. 6 is a view illustrating a step of the method for forming a quantumdot according to a modification of the first embodiment of the presentinvention (Part 1).

FIG. 7 is views illustrating steps of the method for forming the quantumdot according to the modification of the first embodiment of the presentinvention (Part 2).

FIG. 8 is a sectional view of the quantum semiconductor device accordingto a second embodiment of the present invention, which illustrates astructure thereof.

FIG. 9 is a view of an energy band structure of the quantumsemiconductor device according to the second embodiment of the presentinvention.

FIG. 10 is sectional views of the quantum semiconductor device in thesteps of the method for fabricating the quantum semiconductor deviceaccording to the second embodiment of the present invention.

FIG. 11 is a sectional view of the quantum semiconductor deviceaccording to a modification of the second embodiment of the presentinvention, which illustrates a structure thereof.

FIG. 12 is sectional views of the quantum semiconductor device accordingto the modification of the second embodiment of the present invention inthe steps of the method for fabricating the quantum semiconductordevice, which illustrate the method.

FIG. 13 is an upper side view of the quantum semiconductor deviceaccording to the modification of the second embodiment of the presentinvention with the interconnections connected to the electrode pads.

FIG. 14 is a sectional view of the quantum semiconductor deviceaccording to a third embodiment of the present invention, whichillustrates a structure thereof.

FIG. 15 is sectional views of the quantum semiconductor device accordingto the third embodiment of the present invention in the steps of themethod for fabricating the quantum semiconductor device, whichillustrate the method.

FIG. 16 is a sectional view of the quantum semiconductor deviceaccording to a modification of the third embodiment of the presentinvention, which illustrates a structure thereof.

FIG. 17 is sectional views of the quantum semiconductor device accordingto the modification of the third embodiment of the present invention inthe steps of the method for fabricating the quantum semiconductordevice, which illustrate the method.

FIG. 18 is a sectional view of the quantum semiconductor deviceaccording to a fourth embodiment of the present invention, whichillustrates a structure thereof.

FIG. 19 is a view of an energy band structure of the quantumsemiconductor device according to the fourth embodiment of the presentinvention.

FIG. 20 is sectional views of the quantum semiconductor device accordingto the fourth embodiment of the present invention in the steps of themethod for fabricating the quantum semiconductor device, whichillustrate the method.

FIG. 21 is a sectional view of the quantum semiconductor deviceaccording to a modification of the fourth embodiment of the presentinvention, which illustrates a structure thereof.

FIG. 22 is sectional views of the quantum semiconductor device accordingto the modification of the fourth embodiment of the present invention inthe steps of the method for fabricating the quantum semiconductordevice, which illustrate the method.

FIG. 23 is a sectional view of the quantum semiconductor deviceaccording to a fifth embodiment of the present invention, whichillustrates a structure thereof.

FIG. 24 is sectional views of the quantum semiconductor device accordingto the fifth embodiment of the present invention in the steps of themethod for fabricating the quantum semiconductor device, whichillustrate the method.

FIG. 25 is a sectional view of the quantum semiconductor deviceaccording to Modification 1 of the fifth embodiment of the presentinvention, which illustrates a structure thereof.

FIG. 26 is sectional views of the quantum semiconductor device accordingto Modification 1 of the fifth embodiment of the present invention inthe steps of the method for fabricating the quantum semiconductordevice, which illustrate the method.

FIG. 27 is a sectional view of the quantum semiconductor deviceaccording to Modification 2 of the fifth embodiment of the presentinvention, which illustrates a structure thereof.

FIG. 28 is sectional views of the quantum semiconductor device accordingto Modification 2 of the fifth embodiment of the present invention inthe steps of the method for fabricating the quantum semiconductordevice, which illustrate the method.

BEST MODE FOR CARRYING OUT THE INVENTION A First Embodiment

The method for forming a quantum dot according to a first embodiment ofthe present invention will be explained with reference to FIGS. 1 to 5.

The method for forming a quantum dot according to the present embodimentmainly comprises the step of forming an oxide in a dot-shape on thesurface of a semiconductor substrate, the step of removing the oxide toform a concavity in the position where the oxide has been removed, andthe step of growing a semiconductor layer on the semiconductor substratewith the concavity formed in to form a quantum dot of the semiconductorlayer in the concavity.

(a) To Form an Oxide

First, the step of forming an oxide in a dot-shape on the surface of asemiconductor substrate will be explained with reference to FIG. 1. FIG.1 is a view illustrating a step of a method for forming a quantum dotaccording to the present embodiment (Part 1). FIG. 1 is a perspectiveview.

As illustrated in FIG. 1, a semiconductor substrate 10 of, e.g., GaAsheavily doped with an impurity is prepared.

Then, an oxide 12 is formed in a dot-shape on the surface of thesemiconductor substrate 10 by AFM oxidation.

AFM oxidation is a method for forming an oxide on the surface of asample by approaching the probe of an AFM to the sample and applying avoltage between the AFM probe and the sample.

As illustrated in FIG. 1, when the AFM probe 14 is brought near to thesemiconductor substrate 10, a negative bias is applied to the probe 14,and a positive bias is applied to the semiconductor substrate 10, watercontained in the atmospheric air is dissociated as follows near theforward end of the probe 14, and oxidation reaction takes place on thesurface of the semiconductor substrate 10.H₂O→OH⁻+H⁺

When the semiconductor substrate 10 is, e.g., a GaAs substrate, thefollowing oxidation reaction takes place on the surface of thesemiconductor substrate 10.2GaAs+6OH⁻→Ga₂O₃+As₂O₃+3H₂+6e⁻

The oxidation reaction using a GaAs substrate as the semiconductorsubstrate 10 is used here. However, the semiconductor substrate 10 isnot essentially a GaAs substrate. An oxide can be similarly formed onthe surface of a semiconductor substrate of another material.

The AFM probe 14 is stopped when the oxide 12 is formed on the surfaceof the semiconductor substrate 10. This is because the oxidationreaction is caused to take place with the AFM probe 14 stopped, wherebythe formed oxide 12 can have substantially circular plane shape.

The ambient atmosphere for AFM oxidation is, e.g., the atmospheric air.

The humidity of the ambient atmosphere for AFM oxidation is, e.g.,40-60%. However, the humidity of the ambient atmosphere for AFMoxidation is not essentially 40-60% and can be suitably set in a rangewhich can cause the oxidation reaction.

The operation mode of the AFM is, e.g., contact mode. The contact modeis an operation mode in which the probe scans the surface of a sample incontact therewith.

The voltage to be applied between the probe 14 and the semiconductorsubstrate 10 in AFM oxidation is, e.g., 3-7 V.

The period of time of the oxidation reaction is, e.g., 1-20 seconds.

Generally, as the voltage applied between the probe 14 and thesemiconductor substrate 10 is higher, the size of the formed oxide 12tends to be larger. The size of the formed oxide 12 tends to be largeras the oxidation reaction is caused to take place longer. However, whenthe voltage between the probe 14 and the semiconductor substrate 10 istoo high, the size of the formed oxide 12 become non-uniform. On theother hand, when the voltage applied between the probe 14 and thesemiconductor substrate 10 is too low, the oxidation reaction does nottake place. Accordingly, the applied voltage and the oxidation period oftime are set suitably so that the oxide 12 of a prescribed size can beformed.

For example, when the applied voltage is set at, e.g., 5 V, and theoxidation period of time is set at, e.g., 4 seconds, the oxide 12 of,e.g., a 50 nm-diameter can be formed.

When the applied voltage is set to be as low as, e.g., 3 V, and theoxidation period of time is set at, e.g., 4 seconds, the oxide 12 is assmall as, e.g., 20 nm-diameter.

When the probe is formed of a carbon nanotube, the oxide 12 of an about10 nm-diameter, which is very small, can be formed. This is because theprobe 14 formed of a carbon nanotube can have a much fine forward end.

The carbon nanotube is a self-assembled nanostructure which is acylindrical structure formed of carbon atoms.

The thus formed oxide 12 has the substantially lower half buried in thesemiconductor substrate 10 and the substantially upper half exposed outof the semiconductor substrate 10.

In the above, the AFM mode is the contact mode but is not essentiallylimited to the contact mode. The operation mode of the AFM can be setsuitably at any other operation mode as long as the operation mode canperform AFM oxidation. For example, the operation mode may be thetapping mode. The tapping mode is an operation mode in which acantilever supporting the probe is oscillated near the resonancefrequency with the probe scanning the surface of a sample intermittentlyin contact therewith. It is preferable that the applied voltage is sethigher in the tapping mode than in the contact mode.

In the above, the oxidation reaction is caused to take place with theAFM probe 14 stopped. However, in the case that the specifications ofthe AFM do not permit to keep the probe 14 at pause, the scan rate isset to be as low as possible. In the case that the oxidation reaction iscaused to take place with the probe 14 on move, the plane shape of theoxide 12 becomes long narrow. Therefore, the displacement distance ofthe probe 14 in the oxidation reaction is preferably set at, e.g., below5 nm including 5 nm.

FIG. 2 is an AFM image of a state of the surface of the semiconductorsubstrate with the oxides formed in a dot-shape.

The semiconductor substrate 10 was an n⁺ type (001) GaAs substrate withSi doped in the surface in a 2×10¹⁸ cm⁻² concentration. The voltageapplied between the probe 14 and the semiconductor substrate 10 was 4 V.The oxidation period of time for each position was 8 seconds.

As seen in FIG. 2, the oxides 12 are formed in a dot-shape almostequidistantly in prescribed positions.

The plane shape of each oxide 12 is substantially circular.

The diameter of the oxide 12 is substantially uniform, specificallyabout 35-45 nm.

The height of the oxide 12 is substantially uniform, specifically about0.6-0.8 nm.

As described above, in the present embodiment, the oxide 12 is formed onthe surface of the semiconductor substrate 10 by AFM oxidation, wherebythe oxide 12 can be formed in a dot-shape in a prescribed position andin a prescribed size.

(b) To Remove the Oxide

Then, the step of removing the oxide to form a concavity in the positionwhere the oxide has been removed will be explained with reference toFIG. 3. FIG. 3 is views illustrating steps of the method for forming aquantum dot according to the present embodiment (Part 2). FIG. 3 issectional views.

As illustrated in FIG. 3A, the oxide 12 formed on the surface of thesemiconductor substrate 10 is removed by, e.g., chemical etching.

As the etchant, an etchant which can selectively remove the oxide 12without etching the semiconductor substrate 10 is suitably used. Theoxide 12 formed by AFM oxidation is substantially the same as the oxide12 formed by wet oxidation in the usual semiconductor device fabricationprocess. Accordingly, the same etchant as used in the usualsemiconductor device fabrication process is suitably used, whereby theoxide 12 can be selectively etched off without etching the semiconductorsubstrate 10.

When the semiconductor substrate 10 is a GaAs substrate, the etchant is,e.g., diluted HCl.

The etchant is diluted HCl here. However, the etchant is not limited todiluted HCl, and any etchant can be suitably used as long as the etchantcan etch selectively the oxide 12 without etching the semiconductorsubstrate 10. For example, diluted hydrofluoric acid, diluted ammonia(NH₄OH), or others may be used.

The etching period of time may be suitably set so that the oxide 12 canbe selectively etched off without affecting the surface of thesemiconductor substrate 10.

For example, when the semiconductor substrate 10 is a GaAs substrate,and the oxide is selectively etched off by using an etchant ofHCl:H₂O=1:20-1:100, the etching period of time is, e.g., 30 seconds toabout several minutes.

When the concentration of the etchant is not so high, the etching periodof time may not be strictly set, without remarkable problems.

The temperature for etching off the oxide 12 is, e.g., the roomtemperature.

When the oxide 12 is thus removed from the surface of the semiconductorsubstrate 10, a concavity 16 is formed in the position where the oxide12 has been removed. The configuration of the concavity 16 issubstantially the same as that of the oxide 12 which has been buriedthere.

It has been conventionally difficult to form a concavity of, e.g., abelow 40 nm-diameter including 40 nm-diameter. In the presentembodiment, however, as described above, the oxide 12 can be formed in,e.g., a 30 nm-diameter or less, which makes it possible to form theconcavity 16 of, e.g., a below 40 nm-diameter including 40 nm-diameter.

As described above, the voltage to be applied between the probe 14 andthe semiconductor substrate 10 is set to be low, and the oxidationperiod of time is set to be short, the oxide 12 can be formed, in abelow 20 nm-diameter including 20 nm-diameter, whereby the concavity 16of, e.g., a below 20 nm-diameter including 20 nm-diameter can be formed.

As described above, with the probes 14 formed of a carbon nanotube, theoxide 12 can be formed in, e.g., about 10 nm-diameter, whereby theconcavity 16 can be formed in a much fine diameter as fine as, e.g.,about 10 nm-diameter.

FIG. 4 is an AFM image of a state of the semiconductor substrate withthe oxide removed.

The etchant was an etchant of HCl:H₂O=1:50. Conditions for the etchingwere the room temperature and 1 minute.

As seen in FIG. 4, the concavities 16 are formed in the positions wherethe oxides 12 have been removed.

The diameter of the concavities 16 is substantially uniform,specifically about 35-45 nm.

The depth of the concavities 16 is below 0.8 nm including 0.8 nm.

The roughness of the surface of the semiconductor substrate 10 is below0.3 nm including 0.3 nm. This means that the surface of thesemiconductor substrate 10, which should not be etched, has not beenetched.

As described, the oxide 12 formed on the semiconductor substrate 10 canbe selectively removed by chemical etching.

After the oxide 12 has been removed, the etchant staying on the surfaceof the semiconductor substrate 10 is washed away with a deionized water.

Then, for example, nitrogen gas is blown to the surface of thesemiconductor substrate 10 to hereby dry the surface of thesemiconductor substrate 10.

In the above, the oxide 12 formed on the semiconductor substrate 10 ischemically etched off but can be etched off not only by chemicaletching, but also by another method.

For example, the oxide 12 can be selectively removed by, e.g., applyingultrasonic waves as follows.

As means for applying ultrasonic waves, an ultrasonic cleaner is usedhere. However, the means for applying ultrasonic waves is notessentially an ultrasonic cleaner and can be any of a wide variety ofmeans as long as the means can apply ultrasonic waves to thesemiconductor substrate.

First, the semiconductor substrate 10 with the oxide 12 formed on isimmersed in the cleaning vessel (not illustrated) of an ultrasoniccleanser (not illustrated) containing deionized water.

Then, ultrasonic waves are applied to the cleaning vessel. The power ofthe ultrasonic waves to be applied is, e.g., 100 W. The applicationperiod of time of the ultrasonic waves is, e.g., several minutes toabout 2 hours. Thus, the oxide 12 formed on the semiconductor substrate10 is removed.

The mechanism for removing oxide 12 from the surface of thesemiconductor substrate 10 by the application of ultrasonic waves willbe as follows.

That is, the atomic bonding is weaker in the interface between the oxide12 and the semiconductor substrate 10, and when ultrasonic waves areapplied, the oxide 12 is released from the surface of the semiconductorsubstrate 10. On the other hand, the power of the applied ultrasonicwaves, however, is not so high as to break the atomic bonding of thesemiconductor substrate 10 itself. Thus, by applying ultrasonic waves,the oxide 12 can be selectively removed from the surface of thesemiconductor substrate 10.

As described above, the oxide 12 can be selectively removed from thesemiconductor substrate 10 by applying ultrasonic waves.

After the oxide 12 has been removed, nitrogen gas, for example, is blownto the semiconductor substrate 10 to thereby dry the surface of thesemiconductor substrate 10.

(c) To Form a Quantum Dot

Then, the step of growing a semiconductor layer on the semiconductorsubstrate with the concavity formed in to form a quantum dot of thesemiconductor layer in the concavity will be explained with reference toFIG. 3.

First, the semiconductor substrate 10 is loaded into the vacuum chamberof a deposition apparatus and pre-baked, e.g., for 1 hour at about 250°C.

Next, the semiconductor substrate 10 is loaded into the depositionchamber of the deposition apparatus to hetero-grow the semiconductorlayer 18 by, e.g., MBE or MOCVD. For example, the material of thesemiconductor layer 18 can be a material whose band gap is narrower thanthat of the material of the semiconductor substrate 10 and whose latticeconstant is larger than that of the semiconductor substrate 10.

For example, when the material of the semiconductor substrate 10 isGaAs, the material of the semiconductor layer 18 can be, e.g. InGaAs.

When the semiconductor layer 18 is grown in vapor phase on thesemiconductor substrate 10 with the concavity 16 formed in, the growthrate of the semiconductor layer 18 at the concavity 16 tends to befaster than the growth rate of the semiconductor layer 18 in the flatregion of the semiconductor substrate 10 other than the concavity 16.Thus, as illustrated in FIGS. 3B to 3E, the quantum dot 20 of thesemiconductor layer 18 are grown largely in the concavity 16.

As illustrated in FIG. 3E, the upper part of the quantum dot 20 isprojected upward beyond the upper surface of the semiconductor layer 18which has been formed in the flat region of the semiconductor substrate10 except the concavity 16.

The growth rate of the quantum dot 20 tends to be dependent on the sizeof the concavity 16. Accordingly, as the concavity 16 has a largerdiameter, the quantum dot 20 having a larger diameter and higher heighttends to be formed. Accordingly, the diameters of the concavities 16 aresuitably set, whereby the quantum dots 20 having different sizes can besuitably set.

The diameter of the quantum dot 20 tends to be substantially equal tothe diameter of the concavity 16. Accordingly, the growth periods oftime of the quantum dots 20 are suitably set, whereby the quantum dots20 having a uniform diameter and different heights can be suitably set.

FIG. 5 is an AFM image of a state of the semiconductor substrate withthe quantum dots formed on.

The material of the semiconductor layer 18 was In_(0.4)Ga_(0.6)As. Thesemiconductor layer 18 was grown under conditions for growing a 4.4 atomlayer. The temperature inside the deposition chamber was 480° C.

As seen in FIG. 5, the quantum dots 20 are formed in a substantiallyuniform size in the concavities 16 formed in the semiconductor substrate10.

Thus, according to the present embodiment, the quantum dots 20 can beformed in a substantially uniform size in the positions where theconcavities 16 are formed.

After the step of removing the oxide 12 and before the step of formingthe quantum dot 20, a smooth layer (not illustrated) for smoothing therough surface of the semiconductor substrate 10 may be formed. When thesemiconductor layer 18 is formed on the rough surface of thesemiconductor substrate 10, there is a risk that the quantum dot 20 maybe formed on the semiconductor substrate 20 in the region other than theconcavity 16. However, when the smooth layer is formed on thesemiconductor substrate 10, the surface of the semiconductor substrate10 is smoothed, whereby the formation of quantum dots on thesemiconductor substrate 10 in the region other than the concavity 16 canbe prevented. The thickness of the smooth layer may be set suitably notto make the concavity 16 too shallow and may be, e.g., below 1 nmincluding 1 nm.

As described above, according to the present embodiment, the dot-shapedoxide is formed on the surface of a semiconductor substrate by AFMoxidation, and the oxide is removed to thereby form the concavity in thesemiconductor substrate, whereby the concavity can be formed preciselyin a prescribed position and in a prescribed size the Quantum dot isformed in such-formed concavity, whereby the quantum dot of good qualitycan be formed in a prescribed position and in a prescribed size.

According to the present embodiment, the quantum dot can be formed in aprescribed position and in a prescribed size, whereby the quantum dotwhich can be used in the fields of quantum information and quantumcomputation, etc. can be formed. According to the present embodiment,the quantum dot is usable in the quantum computer proposed in, e.g.,Phys. Rev. A 62, 062316, (2000).

(A Modification)

Next, the method for forming a quantum dot according to one modificationof the present embodiment will be explained with reference to FIGS. 6and 7. FIGS. 6 and 7 are views illustrating steps of the method forforming a quantum dot according to the present modification. FIG. 6 is aperspective view, and the FIG. 7 are sectional views.

The method for forming a quantum dot according to the presentmodification is characterized mainly in that a semiconductor layer 22having a smooth surface is formed on the semiconductor substrate 10.

First, as illustrated in FIG. 6, the semiconductor layer 22 of a 300nm-thickness GaAs is formed on the entire surface of the semiconductorsubstrate 10 by, e.g., MBE. The surface roughness of the semiconductorlayer 22 is as small as possible and is preferably, e.g., below 0.5 nmincluding 0.5 nm.

Then, the oxide 12 is formed in a dot-shape on the surface of thesemiconductor layer 22. Conditions for forming the oxide 12 may be,e.g., the same as described above. According to the presentmodification, the semiconductor layer 22 having a smooth surface isformed on the semiconductor substrate 10, and the oxide 12 is formed onthe semiconductor layer 22, whereby even when the surface of thesemiconductor substrate 10 itself is rough, the oxide 12 can be formedin a prescribed size in a prescribed position.

Next, as illustrated in FIG. 7A, the oxide 12 is removed. The oxide 12can be removed in the same way as described above. Thus, concavity 16 ais formed in the semiconductor layer 22 in the position where the oxide12 has been removed.

Then, as illustrated in FIGS. 7B to 7E, the semiconductor layer 18 ishetero-grown. The semiconductor layer 18 can be formed in, e.g., thesame way as described above. Thus, quantum dot 20 of the semiconductorlayer 18 is formed in the concavity 16 a. According to the presentmodification, the semiconductor layer 18 is hetero-grown on thesemiconductor layer 22, whose surface is smooth, whereby even when thesurface of the semiconductor substrate 10 itself is rough, the formationof the quantum dot in the region other than the concavity 16 a can beprevented.

As described above, according to the present modification, thesemiconductor layer 22, whose surface is smooth, is formed on thesemiconductor substrate 10, whereby even when the surface of thesemiconductor substrate 10 itself is rough, the quantum dot 20 can beformed in a prescribed position and in a prescribed size without beinginfluenced by the surface roughness of the semiconductor substrate 10.

A Second Embodiment

The quantum semiconductor device according to a second embodiment of thepresent invention and the method for fabricating the quantumsemiconductor device will be explained with reference to FIGS. 8 to 10.FIG. 8 is a sectional view of the quantum semiconductor device accordingto the present embodiment, which illustrates a structure thereof. FIG. 9is a view of an energy band structure of the quantum semiconductordevice according to the present embodiment. FIG. 10 is sectional viewsof the quantum semiconductor device according to the present embodimentin the steps of the method for fabricating the quantum semiconductordevice, which illustrate the method.

First, the structure of the quantum semiconductor device according tothe present embodiment will be explained with reference to FIG. 8.

A quantum dot layer 114 of a semiconductor including self-assembledquantum dots 112 is formed on a semiconductor substrate 110. The quantumdot layer 114 includes the quantum dots 112 in the form ofthree-dimensionally grown islands self-assembled on the semiconductorsubstrate 110 by S-K mode, and a wetting layer 116 formed on thesemiconductor substrate 10 between the quantum dots 112.

A cap layer, i.e., a semiconductor layer 118 is formed on the quantumdot layer 114. In the semiconductor layer 118 on the quantum dots 112,strains are generated due to the lattice mismatching between thematerial of the quantum dots 112 and the material of the semiconductorlayer 118. In FIG. 8, the regions of the semiconductor layer 118, wherethe strains are generated, are indicated by the dot lines.

On the surface of the semiconductor layer 118 in the positions where thestrains are generated, metal particle-shaped electrodes 122 are formed.

The quantum semiconductor device according to the present embodiment ischaracterized mainly in that the electrodes 122 are formed byself-alignment on the surface of the semiconductor layer 118 inpositions where strains are generated, i.e., the metal particle-shapedelectrodes 122 are formed accurately above the respective quantum dots112.

As described above, because of strains generated in the semiconductorlayer 118 on the quantum dots 112, the electrodes 122 are formedaccurately above the respective quantum dots 112, whereby the quantumdots 112, which have been self-assembled are made electricallyaccessible with precision via such electrodes 122. Also, The quantumdots 112, which have been self-assembled, can be made electricallyaccessible independently of each other.

The energy band structure of the quantum semiconductor device accordingto the present embodiment is as illustrated in FIG. 9.

Then, the method for fabricating the quantum semiconductor deviceaccording to the present embodiment will be explained with reference toFIG. 10.

First, the quantum dot layer 114 is formed on the semiconductorsubstrate 110 by, e.g., MBE (Molecular Beam Epitaxy). In the quantum dotlayer 114, the quantum dots 112 are self-assembled by S-K mode. (seeFIG. 10A). The quantum dot layer 114 is formed of a material whoselattice constant is different from that of the material of thesemiconductor substrate 110 resultantly for large lattice mismatching.For example, when the semiconductor substrate 110 is a GaAs substrate,the quantum dot layer 114 can be formed of, e.g., InAs.

Then, the semiconductor layer 118 is formed on the quantum dot layer 114by, e.g., MBE (see FIG. 10B). The semiconductor layer 118 is formed of amaterial whose lattice constant is different from that of the materialof the quantum dot layer 114 resultantly for large lattice mismatching.When the quantum dot layer 14 is formed of InAs, the semiconductor layer118 is formed of, e.g., GaAs.

In the semiconductor layer 118 on the quantum dots 112, strains aregenerated due to the lattice mismatching with the quantum dots 112.

It is preferable that the semiconductor layer 118 is formed relativelythin so that the strains are generated sufficiently up to the surfacethereof. For example, the growth of the semiconductor layer 118 by MBEis stopped when the semiconductor layer 118 has buried the upper end ofthe quantum dots 112. The thickness of the semiconductor layer 118 is,e.g., below 10 nm including 10 nm, more preferably below 5 nm including5 nm. The reason for forming the semiconductor layer 118 thin will bedetailed.

Next, metal droplets are deposited on the surface of the semiconductorlayer 118 by a droplet epitaxy method to form the metal particle-shapedelectrode 122. The droplet epitaxy method here is the technique fordepositing atoms and molecules of evaporated metal on the surface of amaterial such as e.g., an insulator, a semiconductor or others having asurface energy which is lower than that of a metal to thereby grow fineparticles. In this method, the action which minimizes the energy of asystem takes place, whereby the metal grows in fine particles so as tominimize the surface area of the insulator, metal or others. Theelectrodes 122 can be formed of Ga, In, Al, Au or an alloy of them.

The electrodes 122 can be formed in the epitaxial growth chamber of thedeposition apparatus used in forming the semiconductor layer 118 in astep continuously following the step of forming the semiconductor layer118.

When the semiconductor layer 118 and the electrodes 122 are formed inthe continuous steps, after the semiconductor layer 118 has been formedin the epitaxial growth chamber by MBE, the deposition of thesemiconductor forming the material of the semiconductor layer 118 isstopped.

For example, when a III-V semiconductor has been deposited to form thesemiconductor layer 118, the supply of the V element into the epitaxialgrowth chamber is stopped.

The supply of the V element is stopped, and the metal beams of the IIIelement alone are applied to the surface of the semiconductor layer 118,and metal droplets of the III element are deposited on the surface ofthe semiconductor layer 118.

The metal droplets deposited on the surface of the semiconductor layer118 by the droplet epitaxy method are moved to positions where strainshave been generated in the surface of the semiconductor layer 118.

By cooling following the deposition of the metal droplets, the metaldroplets are solidified, and the particles of the metal are formed.Thus, the metal particle-shaped electrodes 122 are formed accurately inthe positions where the strains are generated in the surface of thesemiconductor layer 118, i.e., on the surface of the semiconductor layer118 above the respective quantum dots 112 buried in the semiconductorlayer 118. That is, the electrodes 122 are formed by self-alignment inthe positions where the strains have been generated in the semiconductorlayer 118 due to the presence of the quantum dots 112.

Conditions for forming the electrodes 122 by the droplet epitaxy methodmay be as follows. The following conditions are for the case that thesemiconductor substrate 110 is a GaAs substrate.

The As partial pressure in the epitaxial growth chamber is set at about10⁻⁷ Torr, which is ignorable. When In or Ga is deposited as the metaldroplets, the substrate temperature is set at, e.g., 100-200° C. When Alis deposited as the metal droplets, the substrate temperature is set at,e.g., 300-400° C. The deposition rate is set at, e.g., 0.5-3 atomiclayers (monolayers)/second, and the total deposition amount is set at,e.g., 1-4 atomic layers.

The electrodes 122 can be set at a prescribed size corresponding to thesize of the quantum dots 112 by suitably setting conditions, such as thedeposition amount, etc., for depositing the metal droplets.

Here, the mechanism for the metal particle-shaped electrodes 122 thusformed by the droplet epitaxy method being formed accurately on thesurface of the semiconductor layer 118 in the positions above thequantum dots 112 will be explained.

Generally in the droplet epitaxy method described above, it is knownthat the positions where the metal droplets are deposited on the surfacedepend on a surface free energy distribution of the surface for themetal droplets to be deposited on during the droplet epitaxy process.

For example, when the surface is homogeneously processed, the metaldroplets are deposited on the surface at random.

In contrast to this, when the surface state is locally modified, themetal droplets are deposited in regions where the surface state isintentionally modified. When the surface state is locally modified, forexample, the surface is locally passivated, impurities are locallydeposited, the surface is locally patterned, or electric field arelocally applied (see, e.g., the specification of U.S. Pat. No.6,383,286, the specification of U.S. Pat. No. 6,242,326, thespecification of U.S. Pat. No. 6,033,972, Japanese published unexaminedpatent application No. Hei 04-245620 (1992), Japanese publishedunexamined patent application No. 2000-315654, etc.).

In the method for fabricating the quantum semiconductor device accordingto the present embodiment, the semiconductor layer 118 is formed thin onthe quantum dot layer 114. Accordingly, strains are generated in thesemiconductor layer 118 on the respective quantum dots 112.

The parts of the surface of the semiconductor layer 118 where thestrains are generated have higher surface energy than the part where thestrains are not generated. Accordingly, the metal droplets by thedroplet epitaxy method are deposited selectively on the surface of thesemiconductor layer 118 in the positions above the quantum dots 112,where the strains are generated.

It is for the following reason that, as described above, thesemiconductor layer 118 is formed as thin as possible. That is, when thesemiconductor layer 118 is formed thick in comparison with the height ofthe quantum dots 112, sufficient strains are not generated up to thesurface of the semiconductor layer 118. Accordingly, the surface energyat the parts of the surface of the semiconductor layer 118, which areabove the quantum dots 112 is not largely changed from that of the restpart, which makes difficult to selectively form the metal droplets.

Thus, by the droplet epitaxy method, the metal particle-shapedelectrodes 122 are formed by self-alignment on the surface of thesemiconductor layer 118 accurately in the positions above the respectivequantum dots 112.

Thus, the quantum semiconductor device according to the presentembodiment is fabricated.

As described above, according to the present embodiment, the metalparticle-shaped electrodes 122 can be formed accurately above therespective quantum dots 122 by the droplet epitaxy method due to thestrains generated in the semiconductor layer 118 on the quantum dots112. This makes the quantum dots 112 electrically accessible withprecision. Furthermore, the respective quantum dots 112 can be madeelectrically accessible independently of each other.

(A Modification)

Next, the quantum semiconductor device according to one modification ofthe present embodiment and the method for fabricating the quantumsemiconductor device will be explained with reference to FIGS. 11 to 13.FIG. 11 is a sectional view of the quantum semiconductor deviceaccording to the present modification, which illustrates a structurethereof. FIG. 12 is sectional views of the quantum semiconductor deviceaccording to the present modification in the steps of the method forfabricating the quantum semiconductor device, which illustrate themethod. FIG. 13 is an upper side view of interconnections connected toelectrode pads.

First, the structure of the quantum semiconductor device according tothe present modification will be explained with reference to FIG. 11.

The quantum semiconductor device according to the present modificationis characterized mainly in that the quantum semiconductor devicecomprises the electrode 122, and interconnection 126 electricallyconnected to the electrode 122.

As illustrated in FIG. 11, a trench 124 is formed in the surface of thesemiconductor layer 118. The trench 124 has one end located near theelectrode 122 formed accurately on the surface of the semiconductorlayer 118 in the position above the quantum dot 112.

In the trench 124, the interconnection 126 is formed, electricallyconnected to the electrode 122.

The quantum semiconductor device according to the present modificationincludes the interconnection 126 electrically connected to the electrode122 as described above, which permits the electrode 122 to beelectrically connected to peripheral circuits for applying voltages tothe quantum dot 112, etc. via the interconnection 126. This morefacilitates the electric access to the quantum dots 112.

Next, the method for fabricating the quantum semiconductor deviceaccording to the present modification will be explained with referenceto FIGS. 12 and 13.

In the same way as illustrated in FIGS. 10A and 10B, the quantum dotlayer 114 and the semiconductor layer 118 are sequentially formed.

Next, on the semiconductor layer 118, a semiconductor dot layer 138 isformed by, e.g., MBE. The semiconductor dot layer 138 is epitaxiallygrown to thereby cause semiconductor dot 136 in the form of athree-dimensionally grown island to be self-assembled by S-K mode (seeFIG. 12A). The semiconductor dot layer 138 formed here includessemiconductor dots 136 self-assembled on the semiconductor layer 118,and a wetting layer 140 formed on the semiconductor layer 118 betweenthe semiconductor dots 136. The semiconductor dots 136 may be quantumdots or anti-dots. The semiconductor dot layer 138 is formed of amaterial whose lattice constant is different from that of the materialof the semiconductor layer 118 resultantly for large latticemismatching. For example, when the quantum dot layer 114 is formed ofInGaAs, and the semiconductor layer 118 is formed of GaAs, thesemiconductor dot layer 138 can be formed of AlInAs.

As described above, the semiconductor dot 136 of the semiconductor dotlayer 138 formed above the quantum dot layer 114 by S-K mode with thesemiconductor layer 118 formed therebetween are formed above the quantumdot 112 of the quantum dot layer 114. That is, the position of thequantum dot 112 of the quantum dot layer 114, which is the first layer,and the position of the semiconductor dot 136 of the semiconductor dotlayer 138, which is the second layer, are vertically in alignment witheach other. The quantum dot 112 and the semiconductor dot 136 are formedthus vertically in alignment with each other, because when asemiconductor dot layer is formed above a quantum dot layer with quantumdots formed on, semiconductor dots, which are quantum dots or anti-dots,tend to be formed in the semiconductor dot layer overlapping with thequantum dots.

Then, the wetting layer 140 of the semiconductor dot layer 138 and thesurface layer of the semiconductor layer 118 are oxidized by AFMoxidation to thereby form a line-shaped oxide 128. The line-shaped oxide128 is formed with one end positioned near the semiconductor dot 136formed above the quantum dot 112.

AFM oxidation is a method for forming an oxide on the surface of asample by approaching the AFM probe to the sample and applying a voltagebetween the AFM probe of the AFM and the sample.

In the present modification, for example, in the atmospheric air of40-60% humidity, the AFM probe 130 is approaching to the wetting layer140 of the semiconductor dot layer 138, a negative bias is applied tothe probe 130, and a positive bias is applied to the semiconductorsubstrate 110. With the biases thus being applied, the probe 130 scansthe wetting layer 140. The probe 130 scans a line over the surface ofthe wetting layer 140 where the line-shaped oxide 128 is to be formed.Thus, the wetting layer 140 of the semiconductor layer 138 and thesurface layer of the semiconductor layer 118 scanned by the probe 130are oxidized to hereby form the line-shaped oxide 128 (see FIG. 12B).

Next, the semiconductor dot 136 and the wetting layer 140 of thesemiconductor layer 138, and the line-shaped oxide 128 are removed byetching. Thus, the trench 124 are formed in the surface of thesemiconductor layer 118 from which the semiconductor dot 136 and thewetting layer 140, and the line-shaped oxide 128 have been removed (seeFIG. 12C). For the semiconductor dot layer 138 of, e.g., InAs, HCl isused as the etchant, whereby the semiconductor dot 136 and the wettinglayer 140, and the line-shaped oxide 128 can be concurrently removed.

Then, in the same way as described above, metal droplets are depositedon the surface of the semiconductor layer 118 by the droplet epitaxymethod. At this time, metal droplets are formed on the surface of thesemiconductor layer 118 in the positions where strains have beengenerated and in the trench 124.

Then, the metal droplets are solidified by cooling following thedeposition, and metal particles are formed. Thus, the interconnection126 in the form of the continuously connected metal particles are formedin the trench 124 together with the formation of the metalparticle-shaped electrode 122 (see FIG. 12D).

Thus, the quantum semiconductor device according to the presentmodification is fabricated.

Hereafter, as illustrated in FIG. 13, the electrode pad 132 electricallyconnected to the interconnection 126 may be formed.

A Third Embodiment

The quantum semiconductor device according to a third embodiment of thepresent invention and the method for fabricating the quantumsemiconductor device will be explained with reference to FIGS. 14 and15. FIG. 14 is a sectional view of the quantum semiconductor deviceaccording to the present embodiment, which illustrates a structurethereof. FIG. 15 is sectional views of the quantum semiconductor deviceaccording to the present embodiment in the steps of the method forfabricating the semiconductor device, which illustrate the method. Thesame members of the present embodiment as those of the quantumsemiconductor device according to the second embodiment and the methodfor fabricating the quantum semiconductor device are represented by thesame reference numbers not to repeat or to simplify their explanation.

First, the structure of the quantum semiconductor device according tothe present embodiment will be explained with reference to FIG. 14.

A quantum dot layer 114 of a semiconductor, including self-assembledquantum dots 112 is formed on a semiconductor substrate 110.

A semiconductor layer 118 is formed on the quantum dot layer 114.

Concavities 134 are formed in the surface of the semiconductor layer 118in the positions above the quantum dots 112. Metal particle-shapedelectrodes 122 are formed in the concavities 134.

The quantum semiconductor device according to the present embodiment ischaracterized mainly in that the electrodes 122 are formed in theconcavities 134 formed in the surface of the semiconductor layer 118 inthe positions above the quantum dots 112.

As described above, because of the concavities formed in the surface ofthe semiconductor layer 118 in the positions above the quantum dots 112,the electrodes 122 are formed accurately above the respective quantumdots 112, whereby the quantum dots 112 can be made electricallyaccessible with precision via such electrodes 122. Furthermore, therespective quantum dots 112, which have been self-assembled, can be madeelectrically accessible independently of each other.

The energy band structure of the quantum semiconductor device accordingto the present embodiment is substantially the same as that of thequantum semiconductor device according to the second embodimentillustrated in FIG. 9.

Next, the method for fabricating the quantum semiconductor deviceaccording to the present embodiment will be explained with reference toFIG. 15.

First, in the same way as in the second embodiment, the quantum dotlayer 114 including the quantum dots 112, and the semiconductor layer118 are sequentially formed on the semiconductor substrate 110 (seeFIGS. 15A and 16B).

Then, the semiconductor dot layer 138 is formed on the semiconductorlayer 118 by, e.g., MBE. The semiconductor dot layer 138 is epitaxiallygrown to thereby cause the semiconductor dots 136 in the shape of athree-dimensionally grown island to be self-assembled by S-K mode (seeFIG. 15C). The semiconductor dot layer 138 formed here includes thesemiconductor dots 136 self-assembled on the semiconductor layer 118,and the wetting layer 140 formed on the semiconductor layer 118 betweenthe semiconductor dots 136. The semiconductor dots 136 may be quantumdots or anti-dots. The semiconductor dot layer 138 is formed of amaterial whose lattice constant is different from that of the materialof the semiconductor layer 118 resultantly for largelattice-mismatching. For example, when the quantum dot layer 114 isformed of InGaAs, and the semiconductor layer 118 is formed of GaAs, thesemiconductor dot layer 138 can be formed of AlInAs.

As described above, the semiconductor dots 136 of the semiconductor dotlayer 138 formed on the quantum dot layer 114 laid on the quantum dotlayer 114 by S-K mode via the semiconductor layer 118 formedtherebetween is formed above the quantum dots 112 of the quantum dotlayer 114. That is, the positions of the quantum dots 112 of the quantumdot layer 114, which is the first layer, and the positions of thesemiconductor dots 136 of the semiconductor dot layer 138, which is thesecond layer, are in vertical alignment with each other. The quantumdots 112 and the semiconductor dots 136 are formed, thus overlappingeach other, because when a semiconductor dot layer is formed on aquantum dot layer with quantum dots formed in, semiconductor dots whichare quantum dots or anti-dots tend to be formed in the semiconductor dotlayer, overlapping the quantum dots.

Next, by AFM oxidation, the semiconductor dots 136 and the semiconductorlayer 118 immediately below the semiconductor dots 136 are oxidized. Forexample, in the atmospheric air of 40-60% humidity with the AFM probe130 approximated to the semiconductor dot 136, a negative bias isapplied to the probe 130, and a positive bias is applied to thesemiconductor substrate 110, for a prescribed period of time (see FIG.15D). For example, when the semiconductor dots 136 are formed of InGaAs,an about 3-10 V voltage is applied. Thus, a dot-shaped oxide 142produced by the semiconductor dot 136 and the semiconductor layer 118immediately below the semiconductor dot 136 being oxidized is formed.Thus, the dot-shaped oxides 142 are formed, partially buried in thesurface layer of the semiconductor layer 118.

It is preferable that the semiconductor dots 136 are formed small so asto facilitate the above-described oxidation. For example, preferably,the semiconductor dots 136 are formed in an about 15-30 nm-size.

Next, the dot-shaped oxides 142 are etched off. The etchant is dilutedHCl, or diluted NH₄OH or others in etching off the dot-shaped oxides 142which are formed by oxidizing the semiconductor dots 136 of, e.g.,InGaAs.

Together with the removal of the dot-shaped oxides 142 by the etching,the wetting layer 140 is also etched off. The dot-shaped oxides 142 orthe wetting layer 140 may be removed first. The dot-shaped oxides 142and the wetting layer 140 may be removed simultaneously by selecting asuitable etchant. For example, by using HCl as the etchant, thedot-shaped oxides 142 containing As and the wetting layer 140 of InAscan be simultaneously removed.

The dot-shaped oxides 142 are thus removed, whereby the concavities 134are formed in the surface of the semiconductor layer 118 in thepositions where the semiconductor dots 136 have been formed, i.e., inthe positions above the respective quantum dots 112 buried in thesemiconductor layer 118 (see FIG. 15E).

In a case that the semiconductor dots 136 are not low enough tocompletely oxidize the semiconductor dots 136 once by AFM oxidation orin other cases, the concavities 134 may be formed in the surface of thesemiconductor layer 118 as follows. That is, the concavities 134 may beformed in the surface of the semiconductor layer 118 by repeating thestep of oxidizing a part of the upper end of the semiconductor dot 136by AFM oxidation and the step of etching off the oxidized part of thesemiconductor dots 136.

Next, in the same way as in the second embodiment, metal droplets aredeposited by the droplet epitaxy method on the surface of thesemiconductor layer 118 with the concavities 134 formed in.

At this time, the metal droplets deposited on the surface of thesemiconductor layer 118 moves to the concavities 134 formed in thesurface of the semiconductor layer 118.

Then, the metal droplets is solidified by cooling after the deposition,and the metal particles are formed. Thus, the metal particle-shapedelectrodes 122 are formed in the concavities 134 (see FIG. 15F).

In the method for fabricating the quantum semiconductor device accordingto the present embodiment, even when sufficient strains are notgenerated in the semiconductor layer 118 on the quantum dots 112, theconcavities 134 are formed in positions above the quantum dots 112 inthe surface of the semiconductor layer 118 as described above, wherebythe electrodes 122 can be formed accurately above the quantum dots 112.Cases that sufficient strains are not generated in the semiconductorlayer 118 on the quantum dots 112 are, e.g., the small latticemismatching between the quantum dots 112 and the layer 118, and thethickness of the semiconductor layer 118 is not so thin as, e.g., 10 nm.

It is preferable that the semiconductor dots 136 are completely oxidizedto form the dot-shaped oxides 142 so that no convexities remain aroundthe concavities 134 so that the metal droplets deposited by the dropletepitaxy method can smoothly move to the concavities 134. For the samereason, preferably, in etching off the wetting layer 140, the wettinglayer 140 is sufficiently etched off so that no convexities of residuesof the wetting layer 140 are formed around the concavities 134.

Thus, the quantum semiconductor device according to the presentembodiment is fabricated.

As described above, according to the present embodiment, the concavities134 are formed in the surface of the semiconductor layer 118 above thequantum dots 112 by etching off the dot-shaped oxide 142 which areformed by oxidizing the semiconductor dots 136 formed above the quantumdots 112, and the metal particle-shaped electrodes 122 are formed by thedroplet epitaxy method, whereby the electrodes 122 can be formedaccurately above the respective quantum dots 112. Thus, the quantum dots112 can be accurately electrically accessible. The quantum dots 112 canbe made electrically accessible independently of each other.

(A Modification)

Next, the quantum semiconductor device according to one modification ofthe present embodiment and the method for fabricating the quantumsemiconductor device will be explained with reference to FIGS. 16 and17. FIG. 16 is a sectional view of the quantum semiconductor deviceaccording to the present modification, which illustrates a structurethereof. FIG. 17 is sectional views of the quantum semiconductor deviceaccording to the present modification in the steps of the method forfabricating the quantum semiconductor device, which illustrate themethod.

First, the structure of the quantum semiconductor device according tothe present modification will be explained with reference to FIG. 16.

As is the quantum semiconductor device according to the modification ofthe second embodiment, the quantum semiconductor device according to thepresent modification is characterized mainly in that the electrode 122and the interconnection 126 electrically connected to the electrode 122are formed.

As illustrated in FIG. 16, the trench 124 is formed in the surface ofthe semiconductor layer 118. The trench 124 has one end located near theconcavity 134 formed in the surface of the semiconductor layer 118accurately in the position above the quantum dot 112.

In the trench 124, interconnection 126 is formed, electrically connectedto the electrode 122.

As described above, the quantum semiconductor device according to thepresent modification includes the interconnection 126 electricallyconnected to the electrode 122, whereby peripheral circuits for applyingvoltages to the quantum dot 112, etc. can be electrically connected tothe electrode 122 via the interconnection 126.

Then, the method for fabricating the quantum semiconductor deviceaccording to the present modification will be explained with referenceto FIG. 17.

The same as illustrated in FIGS. 15A to 15E, the concavity 134 is formedin the surface of the semiconductor layer 118 above the quantum dot 112(see FIG. 17A).

Next, a line-shaped oxide 128 is formed on the surface of thesemiconductor layer 118 by AFM oxidation. The line-shaped oxide 128 isformed with one end thereof located in the position of the concavity 134in the surface of the semiconductor layer 118 (see FIG. 17B).

Next, the line-shaped oxide 128 formed on the surface of thesemiconductor layer 118 is etched off. Thus, the trench 124 is formed inthe surface of the semiconductor layer 118 in the position where theline-shaped oxide 128 has been removed (see FIG. 17C).

Then, in the same way as described above, metal droplets are depositedon the surface of the semiconductor layer 118 by the droplet epitaxymethod. At this time, the metal droplets are formed in the concavity 134in the surface of the semiconductor layer 118 and in the trench 124.

Then, the metal droplets are solidified by cooling after the deposition,and the metal particles are formed. Thus, the interconnection 126 in theform of metal particles formed continuous in the trench 124 is formedtogether with the formation of the metal particle-shaped electrode 122(see FIG. 17D).

Thus, the quantum semiconductor device according to the presentmodification is fabricated.

Following this, the electrode pad 132 may be formed, electricallyconnected to the interconnection 126 in the same way as in themodification of the second embodiment.

A Fourth Embodiment

The quantum semiconductor device according to a fourth embodiment of thepresent invention and the method for fabricating the quantumsemiconductor device will be explained with reference to FIGS. 18 to 20.FIG. 18 is a sectional view of the quantum semiconductor deviceaccording to the present embodiment, which illustrates a structurethereof. FIG. 19 is a view of the band energy of the quantumsemiconductor device according to the present embodiment. FIG. 20 issectional views of the quantum semiconductor device according to thepresent embodiment in the steps of the method for fabricating thequantum semiconductor device, which illustrate the method. The samemembers of the present embodiment as those of the quantum semiconductordevice according to the second and the third embodiment and the methodfor fabricating the quantum semiconductor device are represented by thesame reference numbers not to repeat or to simplify their explanation.

First, the structure of the quantum semiconductor device according tothe present embodiment will be explained with reference to FIG. 18.

A quantum dot layer 114 of a semiconductor, which includesself-assembled quantum dots 112 is formed on a semiconductor substrate110.

An intermediate layer, i.e., a semiconductor layer 118 is formed on thequantum dot layer 114.

On the semiconductor layer 118, a semiconductor dot layer 146 of asemiconductor, which includes self-assembled semiconductor dots 144 isformed. The semiconductor dot layer 146 includes the semiconductor dots144 self-assembled on the semiconductor layer 118 and a wetting layer148 formed on the semiconductor layer 118 between the semiconductor dots144. The semiconductor dots 144 of the semiconductor dot layer 146 areformed on the quantum dot layer 114 in the positions above the quantumdots 112. The semiconductor dots 144 can be anti-dots or quantum dots.

Dot-shaped oxides 150 are formed on the upper ends of the semiconductordots 144 of the semiconductor dot layer 146 by oxidizing thesemiconductor dots 144 partially at the upper ends.

A cap layer, i.e., a semiconductor layer 152 is formed on thesemiconductor dot layer 146 including the semiconductor dots 144 havingthe dot-shaped oxides 150 formed at the upper ends.

Concavities 154 are formed in the surface of the semiconductor layer 152in the positions above the quantum dots 112 and the semiconductor dots144 laid the latter above the former. Metal particle-shaped electrodes122 are formed in the concavities 154.

The quantum semiconductor device according to the present embodiment ischaracterized mainly in that the electrodes 122 are formed in theconcavities 154 formed in the surface of the semiconductor layer 152 inthe positions above the quantum dots 112 and the semiconductor dots 144laid the latter above the former, i.e., the electrodes 122 are formedaccurately above the respective quantum dots 112.

Thus, because of the concavities 154 formed in the surface of thesemiconductor layer 152 in the positions above the quantum dots 112, theelectrodes 122 are formed accurately above the respective quantum dots112, whereby the quantum dots 112, which have been self-assembled, canbe made accurately electrically accessible via the electrodes 122. Therespective quantum dots 112, which have been self-assembled, can be madeelectrically accessible independently of each other.

The quantum semiconductor device according to the present embodiment isalso characterized in that the semiconductor layer 152 in addition tothe semiconductor layer 118 is formed between the quantum dots 112 andthe electrodes 122. Because of the semiconductor layer 152 in additionto the semiconductor layer 118, the metal of the electrodes 122 can beprevented from being diffused to intrude into the quantum dots 112 inthermal processing, etc.

The energy band structure of the quantum semiconductor device accordingto the present embodiment is as illustrated in FIG. 19.

The height and width of the barrier between the quantum dot 112 and theelectrode 122 shown in the energy band structure illustrated in FIG. 19can be set at prescribed values by suitably setting the materialcompositions and thicknesses of the semiconductor layers 118, 152, thematerial composition and size of the semiconductor dot 144, the size ofthe dot-shaped oxide 150, etc.

Next, the method for fabricating the quantum semiconductor deviceaccording to the present embodiment will be explained with reference toFIG. 20.

First, in the same way as in the third embodiment, the quantum dot layer114 and the semiconductor dot layer 146 are formed the latter above theformer by, e.g., MBE on the semiconductor substrate 110 with thesemiconductor layer 118 formed therebetween (see FIG. 20A). Thesemiconductor dots 144 are formed of a material whose lattice constantis different from that of the material of the semiconductor layer 118resultantly for large lattice mismatching, and are formed of a materialhaving a higher ground level energy than, e.g., the semiconductor layer118. In this case, the semiconductor dots 144 are anti-dots. Thesemiconductor dots 144 are formed of such material, whereby theinfluence of the semiconductor dots 144 on the buried quantum dots 112can be reduced. The semiconductor dots 144 are formed of, e.g., InAlAs,InGaAlAs or others.

The semiconductor dots 144 are different from the semiconductor dots 136of the second layer formed in the method for fabricating the quantumsemiconductor device according to the third embodiment and are notformed essentially small.

As described above, the semiconductor dots 144 of the semiconductor dotlayer 146 laid by S-K mode on the quantum dot layer 114 with thesemiconductor layer 118 formed therebetween are formed above the quantumdots 112 of the quantum dot layer 114, as in the third embodiment.

Then, the semiconductor substrate 110 is unloaded out of the epitaxialgrowth chamber.

Subsequently, the upper ends of the semiconductor dots 144 of thesemiconductor layer 146 are partially oxidized by AFM oxidation. Forexample, in the atmospheric air of 40-60% humidity, with the probe 130approached to the semiconductor dot 144, a negative bias is applied tothe probe 130, and a positive bias is applied to the semiconductorsubstrate 110, for a prescribed period of time. Thus, the upper ends ofthe semiconductor dots 144 are partially oxidized, and the dot-shapedoxides 150 are formed (see FIG. 20B).

Then, contaminants adsorbed on the surface and a natural oxide film,etc. formed on the surface are removed, and the semiconductor substrate110 is again loaded into the epitaxial growth chamber of the filmdeposition apparatus. For the removal of the contaminants, etc. thecleaning method using, e.g., atom hydrogen irradiation can be used.

Then, the semiconductor layer 152 is formed on the semiconductor dotlayer 146 by, e.g., MBE. The semiconductor layer 152 is formed of, e.g.,the same material as the semiconductor dots 144 or is formed of amaterial having a larger energy gap than the material of thesemiconductor dots 144.

In the growth of the semiconductor layer 152, the growth rate of thesemiconductor layer 152 on the dot-shaped oxides 150, which isnon-crystalline, is lower than the growth rate of the semiconductorlayer 152 in the rest region. Resultantly, the concavities 154 areformed in the surface of the semiconductor layer 152 above thedot-shaped oxides 150 (see FIG. 20C).

Thus, the concavities 154 are formed in the surface of the semiconductorlayer 152 in the positions above the quantum dots 112 and thesemiconductor dots 144 laid the latter above the former.

Next, metal droplets are deposited by the droplet epitaxy method on thesurface of the semiconductor layer 152 with the concavities 154 formed.As in the third embodiments, the metal droplets deposited on the surfaceof the semiconductor layer 152 move to the concavities 154, as in thethird embodiment.

Subsequently, the metal droplets are solidified by cooling after thedeposition, and the metal particles are formed. Thus, the metalparticle-shaped electrodes 122 are formed in the concavities 44 in thesurface of the semiconductor layer 152 (see FIG. 20D).

Thus, the quantum semiconductor device according to the presentembodiment is fabricated.

As described above, according to the present embodiment, the growth rateof the semiconductor layer 152 being lower on the oxide dots 150 formedby partially oxidizing the semiconductor dots 144 at the upper ends laidabove the quantum dots 112 is used to form the concavities 154 in thesurface of the semiconductor layer 152 in the positions above thequantum dots 112 and the metal particle-shaped electrodes 122 are formedby the droplet epitaxy method, whereby the electrodes 122 can be formedin the positions accurately above the respective quantum dots 112. Thus,the quantum dots 112 can be made accurately electrically accessible. Thequantum dots 112 can be made electrically accessible independently ofeach other.

(A Modification)

Next, the quantum semiconductor device according to a modification ofthe present embodiment and the method for fabricating the quantumsemiconductor device will be explained with reference to FIGS. 21 and22. FIG. 21 is a sectional view of the quantum semiconductor deviceaccording to the present modification, which illustrate a structurethereof. FIG. 22 are sectional views of the quantum semiconductor deviceaccording to the present modification in the steps of the method forfabricating the quantum semiconductor device, which illustrate themethod.

First, the structure of the quantum semiconductor device according tothe present modification will be explained with reference to FIG. 21.

The quantum semiconductor device according to the present modificationis characterized mainly in that, as in the quantum semiconductor deviceaccording to the modification of the second and the third embodiments,the quantum semiconductor device includes the electrode 122 and theinterconnection 126 electrically connected to the electrode 122.

As illustrated in FIG. 21, the trench 124 is formed in the surface ofthe semiconductor layer 152. The trench 124 has one end located near theconcavity 154 formed in the surface of the semiconductor layer 152 inthe position accurately above the quantum dot 112.

In the trench 124, the interconnection 126 is formed, electricallyconnected to the electrode 122.

As described above, the quantum semiconductor device according to thepresent modification includes the interconnection 126 electricallyconnected to the electrode 122, whereby peripheral circuits for applyingvoltages to the quantum dot 112, etc. can be electrically connected tothe electrode 122 via the interconnection 126.

Next, the method for fabricating the quantum semiconductor deviceaccording to the present modification will be explained with referenceto FIG. 22.

In the same way as illustrated in FIGS. 20A to 20C, the concavity 154 isformed in the surface of the semiconductor layer 152 in the positionabove the dot-shaped oxide 150 (see FIG. 22A).

Then, a line-shaped oxide 128 is formed on the surface of thesemiconductor layer 152 by AFM oxidation. The line-shaped oxide 128 isformed with one end located at the position of the concavity 154 in thesurface of the semiconductor layer 152 (see FIG. 22B).

Next, the line-shaped oxide 128 formed on the surface of thesemiconductor layer 152 is etched off. Thus, the trench 124 is formed inthe surface of the semiconductor layer 152, from which the line-shapedoxide 128 have been removed (see FIG. 22C).

Next, in the same way as described above, metal droplets are depositedon the surface of the semiconductor layer 152 by the droplet epitaxymethod. At this time, the metal droplets are formed not only in theconcavity 154 in the surface of the semiconductor layer 152, but also inthe trench 124.

Subsequently, the metal droplets are solidified by cooling after thedeposition, and the metal particles are formed. Thus, theinterconnection 126 of the metal particles continuously connected in thetrench 124 is formed together with the formation of the metalparticle-shaped electrode 122 (see FIG. 22D).

Thus, the quantum semiconductor device according to the presentmodification is fabricated.

Hereafter, as in the modification of the second embodiment, theelectrode pad 132 electrically connected to the interconnection 126 maybe formed.

A Fifth Embodiment

The quantum semiconductor device according to a fifth embodiment of thepresent invention and the method for fabricating the quantumsemiconductor device will be explained with reference to FIG. 23 andFIG. 24. FIG. 23 is a sectional view of the quantum semiconductor deviceaccording to the present embodiment, which illustrates a structurethereof. FIG. 24 is sectional views of the quantum semiconductor deviceaccording to the present embodiment in the steps of the method forfabricating the quantum semiconductor device, which illustrate themethod. The same members of the present embodiment as those of thequantum semiconductor device according to the second to the fourthembodiments and the method for fabricating the quantum semiconductordevice are represented by the same reference numbers not to repeat or tosimplify their explanation.

First, the structure of the quantum semiconductor device according tothe present embodiment will be explained with reference to FIG. 23. Inthe quantum semiconductor device according to the present embodiment aswell as the quantum semiconductor device according to the modificationof the second embodiment, an interconnection is formed, electricallyconnected to an electrode.

As illustrated in FIG. 23, a quantum dot layer 114 of a semiconductor,including a self-assembled quantum dot 112 is formed on a semiconductorlayer 110.

On the quantum dot layer 16, an intermediate layer, i.e., asemiconductor layer 118 is formed. Strains due to the latticemismatching are generated in the semiconductor layer 118 on the quantumdot 112.

A metal particle-shaped electrode 122 is formed on the surface of thesemiconductor layer 118 in the position where the strains are generated.A line-shaped oxide 156 is formed on the surface of the semiconductorlayer 118. The line-shaped oxide 156 has one end located near theelectrode 122.

A cap layer, i.e., a semiconductor layer 158 is formed on thesemiconductor layer 118 with the electrode 122 and the line-shaped oxide156 formed on.

An opening 160 is formed in the semiconductor layer 158 in the positionabove the electrode 122 and down to the electrode 122. In the opening160, an electrode 123 is buried, electrically connected to the electrode122.

A trench 162 is formed in the surface of the semiconductor layer 158along the line-shaped oxide 156.

An interconnection 164 is formed in the trench 162, electricallyconnected to the electrode 123.

As described above, the quantum semiconductor device according to thepresent embodiment is characterized mainly in that the line-shaped oxide156 is formed below the interconnection 164. Because of the line-shapedoxide 156, a barrier is formed against the carriers below theinterconnection 164, and the leak current from the interconnection 164can be suppressed.

Next, the method for fabricating the quantum semiconductor deviceaccording to the present embodiment will be explained with reference toFIG. 24.

First, in the same way as in the second embodiment illustrated in FIGS.10A to 10C, the electrode 122 is formed on the surface of thesemiconductor layer 118 in the position above the quantum dot 112.

Next, the line-shaped oxide 156 is formed on the surface of thesemiconductor layer 118 by AFM oxidation (see FIG. 24A). The line-shapedoxide 156 is formed with one end located near the electrode 122.

Next, the semiconductor layer 158 is formed by, e.g., MBE on thesemiconductor layer 118 with the electrode 122 and the line-shaped oxide156 formed on.

At this time, the semiconductor layer 158 grows on the electrode 122 andthe line-shaped oxide 156 at a lower growth rate than in the restregion. Resultantly, the concavity 165 is formed in the surface of thesemiconductor layer 158 in the position above the electrode 122, and thetrench 162 is formed in the surface of the semiconductor layer 158 alongthe line-shaped oxide 156 (see FIG. 24B).

Next, the semiconductor layer 158 is oxidized by AFM oxidation in theposition where the concavity 165 is formed. The dot-shaped oxide 166 isformed thus by oxidizing the part of the semiconductor layer 158, wherethe concavity 165 is formed (see FIG. 24C). The oxidation by AFMoxidation is set on until the size of the dot-shaped oxide 166 reachesthe electrode 122 below the dot-shaped oxide 166.

Then, the dot-shaped oxide 166 formed in the concavity 165 is etchedoff. Thus, the opening 160 is formed in the semiconductor layer 158 downto the electrode 122 (see FIG. 24D).

Then, metal droplets are deposited by the droplet epitaxy method on thesurface of the semiconductor layer 158 with the opening 160 and thetrench 162 formed in. At this time, the metal droplets are formed in theopening 160, down to the electrode 122, and in the trench 162.

Then, the metal droplets are solidified by cooling after the deposition,and the metal particles are formed. Thus, the metal particle-shapedelectrode 123 electrically connected to the electrode 122 and theinterconnection 164 of the metal particles continuously connected in thetrench 162 are formed (see FIG. 24E).

Thus, the quantum semiconductor device according to the presentembodiment is formed.

Hereafter, as in the modification of the second embodiment, theelectrode pad 132 electrically connected to the interconnection 164 maybe formed.

As described above, according to the present embodiment, theinterconnection 164 is formed in the trench 162 formed in the surface ofthe semiconductor layer 158 in the position above the line-shaped oxide156, whereby the line-shaped oxide 156 can form the barriers to thecarriers below the interconnection 164 and thus leak currents from theinterconnection 164 can be suppressed.

In the present embodiment, the line-shaped oxide 156 is formed in 1layer. However, the line-shaped oxide 156 may be formed in a pluralityof layers by repeating the above described step. The line-shaped oxide156 are formed in a plurality of layer below the interconnection 164,whereby the leak current from the interconnection 164 can be moreeffectively suppressed.

(Modification 1)

The quantum semiconductor device according to Modification 1 of thepresent embodiment and the method for fabricating the quantumsemiconductor device will be explained with reference to FIGS. 25 and26. FIG. 25 is a sectional view of the quantum semiconductor deviceaccording to the present modification, which illustrates a structurethereof. FIG. 26 is sectional views of the quantum semiconductor deviceaccording to the present modification in the steps of the method forfabricating the quantum semiconductor device, which illustrate themethod.

First, the structure of the quantum semiconductor device according tothe present modification will be explained with reference to FIG. 25.

In the quantum semiconductor device according to the presentmodification, the electrode 122 is formed above the quantum dot 112, asin the third embodiment. That is, as illustrated in FIG. 25, theelectrode 122 is formed in the concavity 134 formed in the surface ofthe semiconductor layer 118 in the position above the quantum dot 112.

In the surface of the semiconductor layer 118, a line-shaped oxide 156is formed with one end located near the electrode 122 formed in theconcavity 134.

The semiconductor layer 158 is formed on the semiconductor layer 118with the electrode 122 and the line-shaped oxide 156 formed on.

On the semiconductor layer 158, as described above, the electrode 123electrically connected to the electrode 122 and the interconnection 164electrically connected to the electrode 123 are formed.

Thus, the quantum semiconductor device according to the presentmodification is constituted.

Next, the method for fabricating the quantum semiconductor deviceaccording to the present modification will be explained with referenceto FIG. 26.

First, as in the third embodiment, the electrode 122 is formed in theconcavity 134 formed in the surface of the semiconductor layer 118 inthe position above the quantum dot 112.

Next, the line-shaped oxide 156 is formed by AFM oxidation with one endlocated near the electrode 122 formed in the concavity 134 (see FIG.26A).

Next, the semiconductor layer 158 is formed on the semiconductor layer118 with the electrode 122 and the line-shaped oxide 156 formed on thesurface. Then, the opening 160 is formed in the semiconductor layer 158down to the electrode 122 (see FIG. 26B).

Next, by the droplet epitaxy method, the electrode 122 is formed in theopening 160 down to the electrode 122, and the interconnection 164 isformed in the trench 162 (see FIG. 26C).

Thus, the quantum semiconductor device according to the presentmodification is fabricated.

(Modification 2)

The quantum semiconductor device according to Modification 2 of thepresent embodiment and the method for fabricating the quantumsemiconductor device will be explained with reference to FIGS. 27 and28. FIG. 27 is a sectional view of the quantum semiconductor deviceaccording to the present modification, which illustrates the structure.FIG. 28 is sectional views of the quantum semiconductor device in thesteps of the method for fabricating the quantum semiconductor device,which illustrate the method.

First, the structure of the quantum semiconductor device according tothe present modification will be explained with reference to FIG. 27.

In the quantum semiconductor device according to the presentmodification, as in the fourth embodiment, the electrode 122 is formedabove the quantum dot 112. That is, as illustrated in FIG. 27, theelectrode 122 is formed in the surface of the semiconductor layer 152 inthe position above the quantum dot 112 and the semiconductor dot 144.

In surface of the intermediate layer, i.e., the semiconductor layer 152,the line-shaped oxide 156 is formed with one end located near theelectrode 122 formed in the concavity 154.

The cap layer, i.e., the semiconductor layer 158 is formed on thesemiconductor layer 162 with the electrode 122 and the line-shaped oxide156 formed on.

On the semiconductor layer 158, as described above, the electrode 123electrically connected to the electrode 122 and the interconnection 164electrically connected to the electrode 123 are formed.

Thus, the quantum semiconductor device according to the presentmodification is constituted.

Next, the method for fabricating the quantum semiconductor deviceaccording to the present modification will be explained with referenceto FIG. 28.

First, in the same way as in the fourth embodiment, the electrode 122 isformed in the concavity 154 formed in the surface of the semiconductorlayer 152 in the position above the quantum dot 112 and thesemiconductor dot 144.

Next, the line-shaped oxide 156 is formed by AFM oxidation with one endlocated near the electrode 122 formed in the concavity 154 (see FIG.28A).

Then, the semiconductor layer 158 is formed on the semiconductor layer152 with the electrode 122 and the line-shaped oxide 156 formed on thesurface. Then, the opening 160 is formed in the semiconductor layer 158down to the electrode 122 (see FIG. 28B).

Next, by the droplet epitaxy method, the electrode 123 are formed in theopening 160 down to the electrode 122, and the interconnection 164 isformed in the trench 162 (see FIG. 28C).

Thus, the quantum semiconductor device according to the presentmodification is fabricated.

Modified Embodiments

The present invention is not limited to the above-described embodimentsand can cover other various modification.

For example, in the first embodiment, AFM is used to form the oxide onthe semiconductor substrate, but AFM is not essentially used. That is,any apparatus can be suitably used as long as the apparatus can apply avoltage between a probe-shaped conductor and the semiconductor substratewhich are approached to each other to thereby form the oxide on thesemiconductor substrate.

In the above-described embodiments, the semiconductor substrate is aGaAs substrate. However, the semiconductor substrate is not essentiallya GaAs substrate and can be any other semiconductor substrate. On thesemiconductor substrate with the concavity formed in, a semiconductorlayer of a material whose lattice constant is lager than the material ofthe semiconductor substrate, whereby the quantum dot can be formed inthe concavity.

For example, the semiconductor substrate may be a Si substrate. When aSi substrate is used, the oxide of SiO₂ is formed. In this case, dilutedhydrofluoric acid, for example, is used as the etchant, whereby theoxide of SiO₂ can be selectively etched off without etching the Sisubstrate. A semiconductor layer of a material whose lattice constant islarger than that of Si, is grown on an Si substrate with the concavityformed in, whereby the quantum dot can be formed in the concavity. Thematerial of the semiconductor layer can be, e.g., Ge.

The semiconductor substrate may be an AlGaAs substrate. A semiconductorlayer of a material whose lattice constant is larger than that ofAlGaAs, is grown on an AlGaAs substrate with the concavity formed in,whereby the quantum dot can be formed in the concavity. The material ofthe semiconductor layer can be, e.g., InAlGaAs.

The semiconductor substrate may be a ZnSe substrate. A semiconductorlayer of a material whose lattice constant is larger than that of ZnSe,is grown on a ZnSe substrate with the concavity formed in, whereby thequantum dot can be formed in the concavity. The material of thesemiconductor layer can be, e.g., CdSe.

In the second to the fifth embodiments, the electrodes are formed withrespect to the quantum dots self-assembled by S-K mode. However, thequantum dots with respect to which the electrodes are formed are notessentially self-assembled by S-K mode. For example, the presentinvention is applicable to the case that the electrodes are formed withrespect to the quantum dots vertically laid one above another.

In the second to the fifth embodiments, the quantum dots are oxidized orthe semiconductor layer is oxidized to form the line-shaped oxides, byAFM oxidation. The probe for applying a voltage can be any as long asthe probe is formed of a conductor. For example, as the probe, a carbonnanotube can be also used.

In the second to the fifth embodiments, the quantum dots or thesemiconductor layer is oxidized by AFM oxidation. However, the quantumdots, etc. are not essentially oxidized by AFM oxidation. The quantumdots, etc. can be oxidized by any means as long as the means can oxidizeselectively a fine area.

INDUSTRIAL APPLICABILITY

The method for forming a quantum dot according to the present inventionis useful to form quantum dots usable in the fields of quantuminformation and quantum computation, and other fields. The quantumsemiconductor device according to the present invention and the methodfor fabricating the quantum semiconductor device are useful to realizeaccurate electric access to quantum dots, which is very important invarious aspects of fundamental studies of the quantum dot and theirapplication developments, etc.

1. A quantum semiconductor device comprising: a plurality of quantumdots formed over a semiconductor substrate; a semiconductor layerformed, burying said plurality of quantum dots; a plurality of metalelectrodes formed by self-alignment above positions where strains aregenerated in the semiconductor layer because of said plurality ofquantum dots, said plurality of metal electrodes being formed above eachof said plurality of quantum dots; and a plurality of interconnectionsformed in a plurality of trenches formed in a surface of thesemiconductor layer, each of said plurality of interconnections beingelectrically connected to each of said plurality of metal electrodes. 2.A quantum semiconductor device according to claim 1, wherein thesemiconductor layer is formed of a material whose lattice constant isdifferent from that of said quantum dots.
 3. A quantum semiconductordevice comprising: a plurality of quantum dots formed over asemiconductor substrate; a semiconductor layer formed, burying said aplurality of quantum dots; and a plurality of metal electrodes formed ineach of concavities formed in a surface of the semiconductor layer ineach of positions above each of said a plurality of quantum dots.
 4. Aquantum semiconductor device comprising: a plurality of quantum dotsformed over a semiconductor substrate; a semiconductor layer formed,burying said plurality of quantum dots; a plurality of metal electrodesformed by self-alignment above positions where strains are generated inthe semiconductor layer because of said plurality of quantum dots, saidplurality of metal electrodes being formed above each of said pluralityof quantum dots; a plurality of line-shaped oxides formed on the surfaceof the semiconductor layer, each of said plurality of line-shaped oxideshaving one end located near each of said plurality of metal electrodes;a second semiconductor layer formed, burying said plurality of metalelectrodes and said plurality of line-shaped oxides; a plurality ofsecond metal electrodes buried in said second semiconductor layer, eachof said plurality of second metal electrodes being electricallyconnected to each of said plurality of metal electrodes; and a pluralityof interconnections formed in a plurality of trenches formed in asurface of said second semiconductor layer along each of said pluralityof line-shaped oxides, each of said plurality of interconnections beingelectrically connected to each of said plurality of second metalelectrodes.
 5. A quantum semiconductor device comprising: a quantum dotformed over a semiconductor substrate; a first semiconductor layerformed, burying the quantum dot; a semiconductor dot formed on the firstsemiconductor layer in a position above the quantum dot; a dot-shapedoxide formed of the partially oxidized semiconductor dot; a secondsemiconductor layer formed, burying the semiconductor dot; and anelectrode formed in a concavity formed in a surface of the secondsemiconductor layer in a position above the dot-shaped oxide.
 6. Aquantum semiconductor device according to claim 5, wherein thesemiconductor dot is a quantum dot or an anti-dot.
 7. A quantumsemiconductor device according to claim 5, further comprising aninterconnection formed in a trench formed in the surface of the secondsemiconductor layer and electrically connected to said electrode.
 8. Aquantum semiconductor device according to claim 5, further comprising: aline-shaped oxide formed on the surface of the second semiconductorlayer and having one end located near said electrode; a thirdsemiconductor layer formed, burying said electrode and the line-shapedoxide; another electrode buried in the third semiconductor layer andelectrically connected to said electrode; and an interconnection formedin a trench formed in a surface of the third semiconductor layer alongthe line-shaped oxide and electrically connected to said anotherelectrode.
 9. A quantum semiconductor device according to claim 3,wherein said quantum dots are each formed in a three-dimensionally grownisland self-assembled by S-K mode.
 10. A quantum semiconductor deviceaccording to claim 3, further comprising an interconnection formed in atrench formed in the surface of the semiconductor layer and electricallyconnected to said metal electrode.
 11. A quantum semiconductor deviceaccording to claim 3, further comprising: a plurality of line-shapedoxides formed on the surface of the semiconductor layer, each of saidplurality of line-shaped oxides having one end located near each of saidplurality of metal electrodes; a second semiconductor layer formed,burying said plurality of metal electrodes and said plurality ofline-shaped oxides; a plurality of second metal electrodes buried insaid second semiconductor layer, each of said plurality of second metalelectrodes being electrically connected to each of said plurality ofmetal electrodes; and a plurality of interconnections formed in aplurality of trenches formed in a surface of said second semiconductorlayer along each of said plurality of line-shaped oxides, each of saidplurality of interconnections being electrically connected to each ofsaid plurality of second metal electrodes.
 12. A quantum semiconductordevice according to claim 6, further comprising an interconnectionformed in a trench formed in the surface of the second semiconductorlayer and electrically connected to said electrode.
 13. A quantumsemiconductor device according to claim 6, further comprising: aline-shaped oxide formed on the surface of the second semiconductorlayer and having one end located near said electrode; a thirdsemiconductor layer formed, burying said electrode and the line-shapedoxide; another electrode buried in the third semiconductor layer andelectrically connected to said electrode; and an interconnection formedin a trench formed in a surface of the third semiconductor layer alongthe line-shaped oxide and electrically connected to said anotherelectrode.
 14. A quantum semiconductor device according to claim 5,wherein the quantum dot is formed in a three-dimensionally grown islandself-assembled by S-K mode.
 15. A quantum semiconductor device accordingto claim 6, wherein the quantum dot is formed in a three-dimensionallygrown island self-assembled by S-K mode.